Method and device for detecting fumigants in air samples

ABSTRACT

The present invention relates to a method and a device for detecting fumigants in air samples. The device can be designed for example as a portable analyzer  10  equipped with a stab probe  11  for sampling soil air. There are provided detection means  16  which, upon contact with the air sample, generate electrical signals which depend on the concentration of the fumigants to be detected in the air sample. In accordance with the invention, the detection means  16  comprise at least one mass-sensitive sensor, for example an array of quartz microbalances, which have suitable selective surface layers.

The present invention relates to a method and device for detectingfumigants in air samples.

The soils used in agriculture or horticulture can be infested withplant-injurial organisms, also known as phytopathogens, such asnematodes, soil-dwelling insects, germinating plants, soil bacteria orsoil fungi. For example, the yield loss resulting from the attack ofuseful plants by soil-dwelling nematodes or root-knot nematodes areestimated at approx. 12% worldwide, which corresponds to a loss ofincome for the producers of at least 7 billion US dollars. Moreover,international plant quarantine regulations stipulate that the plantmaterial being exported and imported be free from nematodes but alsothat it be grown in nematode-free soils. It is therefore frequentlynecessary to disinfest/disinfect agriculturally used soils before thenext planting or replanting, for example by treatment with a fungicideor a nematicide.

In most cases, what are known as fumigants (smoke generators orgas-generating products for the soil) are employed for soildisinfestation/disinfection. Fumigants are usually applied in liquidform or in solid form. While liquid formulations act in the soil owingto their high vapor pressure, solid compounds which are introduced intothe soil in the form of, for example, granules, disintegrate in thepresence of soil moisture into gaseous, biocidally active compounds. Thepreparations diffuse through the soil's capillary system, where theycome into contact with the pests and act as a respiratory poison.Fumigants may also act as contact poisons upon direct contact.

In recent decades, methyl bromide was the most widely usedgas-generating product for the soil. However, methyl bromide is known tobe a substance which contributes to damaging the earth's ozone layer.This is why, in 1997, over 100 countries decided, in a follow-upconference to the “Montreal Protocol on Substances that Deplete theOzone Layer” that methyl bromide must no longer be employed from 2005 inindustrial states and from 2015 in developing countries. In 1998, the USCongress, in harmonization with the “Montreal Protocol”, postponed until2005 a ban of methyl bromide already intended to be in force in 2001.

This is why other soil disinfestants/disinfectants are increasinglybeing considered in place of methyl bromide.

Thus, U.S. Pat. No. 2,838,389 describes the use oftetrahydro-3,5-dimethyl-1,3,5-thiadiazine-2-thione, of the formula

common name “dazomet” as a product for the disinfestation/disinfectionof soils in agriculture and horticulture. When dazomet is applied,methyl isothiocyanate (MITC), of the formulaMe—N═C═S,is released as the actual bioactive agent. Dazomet itself therefore onlyconstitutes what is known as a prodrug precursor. In the more recentliterature, the soil disinfestation/disinfection with dazomet in theform of Basamid® granules (Basamid is a registered trademark of BASF AG,Ludwigshafen) is described in Forest Prod. J. 43(2) (1993), pp. 41-44;in Acta Horticulture 382 (1995) pp. 110 et seq. and in Fand FiberScience 27(2) (1995), pp. 183-197.

Basamid® granules are a relatively inert solid material which becomesactive only upon application to the damp soil. Preferred fields ofapplication for Basamid® are the production of ornamentals, vegetablesand tobacco, tree nurseries, fruit, wine and hop production, and thedisinfestation/disinfection of compost and greenhouse soil. Basamid®granules, which comprise 98 to 100% dazomet, are applied by spreadingand are incorporated into the soil at a depth of 20 to 30 cm, but, ifrequired, also up to a depth of 50 cm. The soil is subsequently usuallycovered with plastic sheeting to keep it damp during the treatment timeand to avoid unduly early losses of the gaseous active component.

Another liquid soil disinfestant/disinfectant which, like dazomet,releases MITC is sodium methylcarbamodithionate, which is sold under thetrade names VAPM, METHAMFLUID, AAMONAN or DICID. Also, MITC isfrequently employed directly or as a formulation under the trade namesTRAPEX and VORAX for gassing. It may also be used as a mixture withmetham. The agent is typically employed as a 20% strength solution in anorganic solvent, which is intended to enhance the volatility of theactive ingredient.

Another liquid soil disinfestant/disinfectant and nematicide is1,3-dichloropropene (1,3-DCP), which is sold under the trade names DCP50, SCHELL-DD, TELONE or DI-TRAPEX and which may also be employed incombination with MITC.

The known gas-generating products for the soil exhibit highphytotoxicity. They can therefore be employed only where the crops havealready been cleared from the area to be treated. After each use of asoil disinfestant/disinfectant, a certain time must elapse before newuseful plants and crop plants can be sown or planted in order to ensurethat there is no danger of adverse effects on the newly-sown ornewly-planted crops.

However, relatively little is known about the activation, release anddegradation of the bioactive substances of the above-mentionedgas-generating products for the soil. Important factors which affect themetabolism of the gas-generating products in the soil are, however,temperature, water content and pH of the soil. The soil type too, forexample the presence of transition metals, plays a certain role in thiscontext.

Likewise, the application of fertilizers and other substances has aconsiderable effect on the rate at which gas-generating products for thesoil are released from the soil. Thus, it has only been shown recentlythat fertilizers comprising thiosulfates reduce the volatility of1,3-DCP, which is normally high (Gan et al., Journal of EnvironmentalQuality 29(5) (2000), pages 1476-1481).

Owing to this multiplicity of factors, the MITC or 1,3-DCP concentrationin the soil as a function of time in the case of a specific applicationcannot be estimated with a sufficient degree of accuracy unlessmeasurements are carried out.

However, not only the knowledge of the exact residual MITC and 1,3-DCPcontent in the soil is important for the user, owing to thephytotoxicity of the substances, but monitoring the room air and ambientatmosphere is necessary for work safety reasons, owing to the toxicaction of these substances. Thus, it has been shown that, when dazometgranules are employed, MITC is released very rapidly at high soiltemperatures above 30° C. and sufficiently high soil moisture. Inparticular in the case of greenhouse applications, it has been observed,when the granules were incorporated into the soil, that the MITCreleased can lead to temporary irritations of the mucous membrane andeyes when ventilation and use are inadequate. In this context, it isalso known that metam fluid or metam-sodium can only be used withinlimits in the greenhouse since they tend to evolve gas very rapidly andin great quantities or that they are no longer approved for suchapplications as is the case for example in California. Without a rapidand reliable determination method there exists therefore the risk ofagricultural workers and other users being exposed to the MITC or1,3-DCP released for an unnecessarily long time or to an unduly highconcentration of these gases.

A variety of HPLC and GC methods are known as analytical orchromatographic methods for detecting the soil gases MITC and/or1,3-dichloropropene (cf., for example, Subramanian, Environm. Toxicol.Chem. 15 (1996), pages 503-513). Thus, for example, the detection of theexposure of agricultural workers in potato production to the nematicidecis-1,3-dichloropropene (cis-DCP) and the determination of its effect onhumans have hitherto been carried out by urine analyses. Only recentlyhas it been found in a Dutch study that the legal upper limit of thedaily exposure was exceeded on over 20% of the days during theobservation period (Brouwer et al., Occupational and EnvironmentalMedicine, 57(11) (2000), pages 738-744).

Chemical, in particular wet-chemical, organic and inorganic detectionmethods of isothiocyanates are likewise known. Thus, Bull. Chem. Soc.Jap. 52, (1979), pages 2155-2156 discloses the detection ofisothiocyanates with iodine monochloride by the formation ofalkylthioureas by reacting the isothiocyanates with alkylamine.

However, these detection methods are either not sensitive enough or socomplicated that they can only be carried out in well-equippedlaboratories. These methods are therefore not suitable for theday-to-day use by the user of crop protection products.

After the application of metam-sodium, 1,3-dichloropropene and/ordazomet has been applied, however, it is particularly important for theuser to test for the presence of residual amounts of MITC and/or 1,3-DCPin the soil which have not yet been mineralized or degraded. If this isthe case, the user must wait for a certain time to elapse since residualamounts of these materials in the soil can lead to plant damage when newuseful plants and crop plants are sown or planted. When using the solidproduct dazomet, in particular, the detection of MITC is particularlyimportant since as has already been described above the release andmineralization of MITC is greatly affected by soil structure,temperature, moisture, type and concentration of fertilizer and otherfactors.

As a rule, users currently employ a biotest system with cress or tobaccoseeds, which has been developed specifically for on-site application. Itis based on the fact that these seeds are inhibited from germinating byminute amounts of MITC and therefore indirectly indicate residual MITC.However, this test, which is known as the cress test, is complicated andrequires, in turn, several days to elapse until it can be demonstratedthat, for example, the cress emerges, that is to say establishes itself.A further disadvantage of such biotests is the false-positive orfalse-negative detection or indication of MITC for example by flawedwater management.

From international patent application WO99/66304 a method and a devicefor trace level detection of analytes using artificial olfactometry isknown. While being preliminarily concerned with medical applicationssuch as detecting halitosis and periodontal disease, numerous otherfields of applications, including detection of fumigants, and numerousdifferent sensor types are briefly mentioned. However, except fromsuggesting to provide a fluid concentrator, WO99/66304 does not discloseany technical teaching how trace levels of analytes can reliably bedetected in air samples. Especially, said document is not concerned withdetection of MITC or 1,3-DCP.

A biosensor for the sensory detection of methyl isothiocyanate (MITC)has already been developed (Iwuoha et al. Analytical Chemistry, 69(8)1674-1681 (1997)). This biosensor employs amperometric enzyme electrodeswhich operate in liquid organic phases (OPEEs=organic phase enzymeelectrodes). Owing to their availability, these compounds which qualifyas substrates for oxidoreductases can be detected in a suitable solvent,i.e. in liquid phase, without requiring complicated sample preparation.Prior to the availability of OPEEs, it was only possible to detect thoseanalytes which are soluble in water. However, the sensor signalsdelivered by the OPEEs are considerably weaker than the sensor signalsyielded in water by electrodes which have natural enzymes immobilized onthem. According to Iwuoha et al. (above), the sensitivity of the OPEEscan be improved in two ways: either natural enzymes are used in asolvent mixture composed of an organic medium (for example acetonitrile,CH₃CN), and water, or the natural enzymes are modified by altering theiramino acid units, i.e. artificial enzymes are used. Each of twobiosensor types described by Iwuoha et al. uses one of the twoprinciples. Horseradish peroxidase (HRP) acts universally as basalenzyme. This enzyme reacts with H₂O₂ and gives rise to a sensor signal.However, since MITC inhibits HRP, an existing concentration of MITCalters the response behavior of the suitably coated biosensor towards aparticular H₂O₂ concentration. Modified enzymes which are adapted forthis purpose are either an HRP which is modified with thehomobifunctional agent suberic acid bis(N-hydroxysuccinimidate) to giveSA-NHS, or an enzyme modified with ethylene glycolbis(N-hydroxysuccinimidate) to give EG-NHS. The electrodes were preparedwith the aid of an electrostatic complexing technique described in theliterature.

However, the known MITC biosensor is not suitable for use inagricultural and horticultural practice since the gaseous analyte mustfirst be dissolved in a suitable solvent and then, in dissolved form,makes contact with the actual sensor. Such a sensor is not onlyexpensive to prepare, owing to the containers required for fresh andused solvents, for wash liquids and purge gases and the correspondingcomplicated arrangement of lines, but is also very heavy and verycomplicated and delicate to handle, so that it is not suitable for amobile analytical device. Moreover, the detection sensitivity which canbe achieved is too low for use in agriculture and in horticulture.

It is an object of the present invention to address the technicalproblem of providing a simple and reliable method of detecting fumigantsin air samples, which method can be employed on site by the user of soildisinfestants/disinfectants, be it in agriculture or in market gardens,and which is sufficiently sensitive for reliably and accuratelydetecting residual amounts of fumigants in soil air or ambient air. Inparticular, it is intended that, by using the method according to theinvention, risks arising in new sowings or new plantings of usefulplants and crop plants, for example following the application of Basamidor 1,3-dichloropropene, are avoided without the user having to resort tothe complicated and time-consuming cress test which has hitherto beenused. Also, it is intended to provide a method of controllingphytopathogens with soil disinfestants/disinfectants which permit a morereliable and more controlled use of the soildisinfestants/disinfectants. Finally, it is intended to provide aninexpensive and compact device for detecting fumigants in air sampleswhich device can be used without knowledge of analytical chemistry andwhich can be employed as a light and compact analytical portable device.

We have found that this object is achieved by measuring fumigants in airsamples by means of chemosensors. Chemosensors are sensors which converta measurable variable which is specific for a chemical substance into asignal, in particular an electrical signal, capable of evaluation. Suchsensors can be based on different physical principles of measurement.For example, a semiconductor element, such as a metal oxidesemiconductor (MOS) or a metal oxide semiconductor field effecttransistor (MOSFET), but also electrically conductive polymers which areknown as conducting polymer sensors (CPS) may act as the sensitivesensor element. Surprisingly, it has now been found that mass-sensitivesensors which are coated with a surface layer which is selective for thefumigants to be detected, are very especially suitable for detectingfumigants in air samples at the sensitivity level required for use inagriculture and horticulture.

Mass-sensitive sensors are known, for example, as “quartzmicro-balances” (QMBs) or as “surface acoustic-wave devices” (SAWs).Quartz micro-balances are employed for example in coating plants, forexample sputtering plants, for monitoring the coating thickness.Usually, a crystal oscillator is integrated into an electrical resonantcircuit. The quartz crystal makes contact with metal electrodes and,exploiting the reverse piezoelectric effect, stimulated at a frequency,typically in the radio frequency range, which corresponds to amechanical natural frequency of the crystal. This results in thestimulation of resonant oscillations, which determine a stableoscillation frequency of the resonant circuit. The resonant frequencynow depends on the mass of the crystal oscillator, so that mass changes,for example owing to absorption or adsorption of an analyte, can bedetected as changes in the resonant frequency. Frequency changes in therange of 1 Hz can be measured by electrical bridge circuits.

Thus, the present invention relates to a method of detecting fumigantsin air samples, wherein an air sample is drawn in, the drawn-in airsample makes contact with at least one mass-sensitive sensor, saidmass-sensitive sensor being coated with a surface layer which isselective for the fumigants to be detected, mass changes of the sensorare detected in the form of electrical signals, and the electricalsignals are evaluated.

In accordance with the invention, fumigants can be detected, for examplein soil air from soils which have been treated with soildisinfestants/disinfectants. To monitor the exposure of agriculturalworkers or workers in greenhouses, the mass-sensitive sensor can also beused for detecting fumigants in air in the vicinity of the soil,preferably up to a level of 1 to 3 m above ground or in room air, inparticular in greenhouses.

The mass-sensitive sensor is especially preferably used for detectingMITC and/or 1,3-dichloropropene (1,3-DCP). However, the mass-sensitivesensor can also be used for detecting other gaseous fumigants, inparticular for detecting methyl bromide.

The sensor is equipped with a coating which is as selective as possiblefor the substance to be detected, for example MITC. Ideally, therefore,a single sensor with a highly-specific coating will suffice to detectthe substance of interest. However, a multiplicity of differentsubstances is present in air samples, in particular soil air from theagricultural sector. Besides the gases present in atmospheric air, soilair shows increased CO₂ contents (typically 0.3-3.0, in some cases up to10% by volume), mainly as a consequence of the degradation bymicroorganisms of degradable organic substances. In addition, othergases are also formed in soils, mainly by microbial processes. Dependingon the substances present, and the Eh-pH conditions of the varioussoils, which differ from season to season, these are, for example, N₂O,NO, NO₂, NH₃, SO₂, H₂S, CH₄, C₂H₄, and other substances with arelatively high vapor pressure. Moreover, depending on the degree ofpollution of the ambient air and of the soils, the presence of volatileorganic compounds such as fuels, solvents and others from anthropogenicsources must be expected to be found in the soil air. Approximately 15years ago, tetrachloroethylene contents in the soil air of from 0.1 to112 μg/m³ and similarly increased trichloroethene and trichloroethanecontents were measured even in largely unpolluted soils in southernGermany.

Usually, coated mass-sensitive sensors exhibit a more-or-less pronouncedsensitivity for individual, but normally several, components of a gasmixture. Chemosensors frequently exhibit a similar degree of sensitivityto substances of a related nature owing to what is known as crosssensitivities. To differentiate between these substances, or to detectunambiguously just a single chemical compound, a plurality of sensors ina suitable combination, known as sensor arrays, are therefore usuallyrequired. Such systems based on a wide range of chemosensory measuringprinciples have already been described in the literature under the name“electronic noses” for applications other than the detection offumigants.

Therefore, it will be preferred in accordance with the invention to usea plurality of sensors which are preferably coated with differentselective layers. In principle, the less specific the coatings ofindividual sensors are for the substances to be detected and the broaderthe range of application of the sensor array is to be, the more sensorswill be required. To detect fumigants, in particular to detect a fewsubstances such as, for example, MITC or 1,3-DCP, preferably two totwelve, in particular approximately six, different sensors are employed.

However, the selective coatings described hitherto in the literature areeither unsuitable or not sensitive enough for detecting fumigants in airsamples. Selection criteria for the choice and combination of suitablelayers are described, for example, by: Nakamura et al., Sensors andActuators B 69/3, 295-301 (2000). Furthermore, piezoelectric sensorsystems with gold electrodes and AT-cut quartz crystals are describedwhich are coated with polydimethylsiloxane, polyetherurethane,polyethylcellulose or polycyanopropylmethylsiloxane and operated as anarray composed of 4 piezoelectrical sensors. The crystals were locatedin a chamber which can be maintained at constant temperature and wasequipped with a gas inlet and outlet. Data gathering, the gas mixtureand its flow through the chamber were controlled by a personal computer.Standardization of the concentrations of the gases in the chamber and ofthe sensitivity of the sensors was performed by isothermal exposure atroom temperature. The concentrations of the gases were 100 to 1 000 ppmin the case of toluene and chloroform and 250 to 2 000 ppm in the caseof n-octane.

Surprisingly, it has now been found that a coating of the mass-sensitivesensor with macrocycles and/or dendrimers is a particularly suitableselective coating for detecting fumigants. Such coatings have alreadybeen described for example for the gravimetric detection of solventvapors by Ehlen et al., Angew. Chem., Int. Ed. English 32, 111-112(1993). Moreover, such selective coatings have been used for detectingcarbonyl compounds in the gas phase, and ammonia.

The preferred field of application of the sensor according to theinvention is the agricultural sector, where it is also intended to beused by operators less versed in the art of instrumentation. It istherefore desirable that the mass-sensitive sensor be especially robust,easy to handle and inexpensive. To allow highly-sensitive measurementsto be performed nevertheless, the fumigants to be detected will, in anespecially preferred embodiment of the method according to theinvention, first be concentrated before making contact with themass-sensitive sensor. To this end, for example, the soil gas can firstbe passed through an adsorber or absorber, for example made of silicagel or “TENAX”, or else the analyte is first condensed and subsequentlydelivered to the sensor in concentrated form with the aid of an inertgas (for example air or nitrogen).

Advantageously, the moisture content of the air sample will additionallybe determined, so that, for example in the case of unduly dry air,indications are obtained that the conversion of dazomet in the soil isinsufficient. In the case of unduly dry air, for example, it is possibleto indicate, by means of a signal, that the reading may be unreliablebecause more, unreacted dazomet may still be present in the soil.

To measure soil-air samples, for example, a probe can be inserted intothe soil to a depth of several centimeters to several tens ofcentimeters, and a specific amount of air can be drawn in via the probeby means of a suction pump and passed past the mass-sensitive sensor.The substance to be detected is deposited on the mass-sensitive sensorand alters the resonant frequency of a resonance circuit into which thesensor is integrated.

According to another embodiment of the invention, a sample of soil to betested, e.g. several 100 g, is filled into a vessel. Subsequently, airwhich includes fumigants released from the soil sample is drawn from thevessel into a measuring chamber which houses the sensor array.

In order to increase the accuracy of measurement, it is possible toremove moisture from the air sample before the air sample is drawn intothe measuring chamber.

The present invention also relates to a method of controllingphytopathogens, which comprises treating the soil with an effectiveamount of a gas-generating product for the soil and subsequentlydetecting released fumigants by the above-described method using amass-sensitive sensor.

The present invention further relates to a device for detectingfumigants in air samples with sampling means for taking an air sample,detection means which, upon contact with the air sample, generateelectrical signals which depend on the concentration of the fumigants tobe detected in the air sample, evaluation means which calculate theconcentration of the fumigants present in the air sample on the basis ofthe electrical signals produced by the detection means, wherein thedetection means of the device comprise at least one mass-sensitivesensor which is coated with a surface layer which is selective for thefumigants to be detected.

The mass-sensitive sensor can comprise for example a surface acousticwave device. While changes in the mass occupancy can be measured verysensitively using surface acoustic wave devices, such sensors are alsohighly sensitive to temperature, so that extensive measures must betaken for maintaining a constant temperature of the resonators. Surfaceacoustic wave devices are therefore less suitable for the field ofapplication in agriculture which is preferred for the purposes of thepresent invention. At least one quartz microbalance is thereforeespecially preferably employed as mass-sensitive sensor. The crystalresonator constitutes a piezoelectric resonator in an electricalresonant circuit. Changes in the mass in the resonator lead to a shiftin resonant frequency of the resonant circuit, which can be evaluatedelectronically.

An array of mass-sensitive sensors of which at least one is a sensorcoated with a surface layer which is selective for the fumigants to bedetected is preferably used. Preferably, however, two or more sensorsare coated with different selective layers and the signals obtained areevaluated by what are known as chemometrical methods.

The surface layer which is selective for the fumigants to be detectedpreferably comprises macrocycles, e.g. lactame amide type macrocycles,and/or dendrimers. It was found that by suitably tailoring the molecularcomposition of said macrocycles and/or dendrimers, a high selectivity ofthe layer for incorporation/adsorption of MITC or 1,3-DCP can beobtained.

In accordance with a preferred variant of the device according to theinvention, at least one concentration unit for the fumigants to bedetected is additionally provided.

Preferably, a moisture or vapor barrier is arranged in the fluid pathwayupstream of the measurement chamber which is permeable for fumigants tobe detected and impermeable for moisture contained in the air sample.Preferably, said moisture barrier comprises a linear low densitypolyethylene (LLDPE) foil. In an embodiment which includes a samplevessel for soil to be tested, the moisture barrier may be a LLDPE bag orin-liner which may be filled with soil. Typically, the thickness of theLLDPE foil will be between 5 and 50 μm, preferably 5-10 μm. Thus thedisposable LLDPE bag will not only function as a moisture barrier butwill also allow the sample vessel (e.g. a box made of stainless steel oranother surface which does not retard fumigants) to remain clean aftereach use.

The detection device according to the invention can be produced to beespecially robust and inexpensive and can also be designed together witha concentration unit to give a very compact device. In particular aportable analyzer can therefore be realized with the device according tothe invention. For use in agriculture, for example, the analyzer cancomprise a rod-like probe which can be stabbed into the soil forsampling.

Stationary analyzers which comprise transmitters which transmitreadings, preferably by air, to a central data acquisition station, mayalso be designed.

The invention is described in greater detail hereinbelow with referenceto the use example shown in the appended drawings.

In the drawings:

FIG. 1 shows a schematic representation of a portable analyzer accordingto the invention for detecting fumigants in the soil air;

FIG. 2 shows a schematic representation of the measuring chamber of theanalyzer of FIG. 1;

FIG. 3 shows a schematic representation of two mass-sensitive sensorelements with selective surface layer as are used in the measuringchamber of FIG. 2;

FIG. 4 shows an example of a synthetic principle for supramolecularunits for selective surface layers;

FIG. 5 shows a schematic synthetic strategy for macrocycles of selectivesurface layers;

FIG. 6 shows an example of a macrocycle which is suitable for preparinga selected surface layer;

FIGS. 7-12 shows examples of dendrimers which are suitable for thepreparation of selective surface layers.

FIG. 1 shows an embodiment of the device according to the invention fordetecting fumigants in air samples. The device according to theinvention is designed as a portable stab probe 10. The probe 10 has ashaft 11 which, with its tip 12, can be inserted into the soil B forsampling soil air. A collar-shaped sheet 13 which, upon insertion of theshaft into the soil, acts as a step is provided at the externalcircumference of the shaft 11 so that sampling is always effected at adefined depth. Provided within the shaft 11 is a line 14 which opens outinto the external circumference of the shaft in the tip 12. The orificeof line 14 is covered by a fine mesh 15 which prevents soil or othersolid particles being drawn in when soil air is drawn in. Line 14 leadsto detection means 16, which comprise a measurement chamber in whichthere is arranged a quartz microbalance array for detecting fumigants. Apreferred embodiment of the detection means 16 is illustrated in greaterdetail further below with reference to FIG. 2, which is shown in greaterdetail. An outlet line 17, which leads out of the detection means 16,opens into conveying means 18, which may be designed as, for example, ablower or a suction pump, and which convey the soil air via line 14through the detection means 16. Moreover, one or more cartridges 19 withpurging and/or calibrating gases can be provided. In the example shown,power supply means such as, for example, batteries 21, which supply theprobe with electricity are arranged in a handle 20 on the probe 10.

The probe can exhibit suitable display units for visually and/oracoustically indicating the readings. Moreover, transmitting andreceiving units for remote control and data transfer may be provided.Such units are known to the skilled worker and are not illustrated ingreater detail in this context.

FIG. 2 is a schematic representation of a preferred embodiment of thedetection means 16 with mass-sensitive sensors. As a matter of course,the detection means 16 can also be used when a sample vessel (notdepicted) for soil is employed. The detection means 16 comprise ameasuring chamber 22, which, in the example shown, is delimited aboveand below by crystal lamellae 23, 24. A plurality of metal spots 25,which define the individual sensors of the array, are vapor-deposited onthe crystal lamellae. The individual sensors are coated with selectivesurface layers, for example by means of electrostatic spraying. Some ofthe coatings preferably have an especially high selectivity for thefumigants to be detected. One or more sensors may also be coated with amaterial which is insensitive to the fumigants to be detected, butespecially sensitive to the moisture present in the air. A preferredmoisture-sensitive material is, for example, polyvinylpyrrolidone (PPy).One or more sensors may also be uncoated or provided with an especiallyinert coating which is sensitive neither to moisture nor to othercomponents of the soil gas. Such sensors are in particular suitable formonitoring any drift in the evaluation electronics, as can occur forexample owing to variations in temperature. A more detailedrepresentation of such a sensor system, in particular a description ofadvantageous measuring modes when determining the concentration of gasesis found in Boeker et al. Sensors and Actuators B 70 (2000), 37-42. Theconstruction of the individual sensor elements is illustrated in greaterdetail hereinbelow in connection with the representation of FIG. 3.

In the example shown in FIG. 2, a concentration unit 26 is arrangedupstream of the measuring chamber 22. If, for example, it is onlyintended to determine the moisture in the soil, or if the fumigants tobe detected are present in higher concentrations, the concentration unitcan also be bypassed via a bypass 27. The flow routes are controlled viasuitable valve units 28, 29, which are controlled automatically by acontrol unit (not shown). To carry out a measurement with upstreamconcentration, soil air is first conveyed via the line 14 through theconcentration unit 26 and the line 30 to the pump 18 (not shown in FIG.2). The connecting line 31 to the measuring chamber 22 is shut. Theconcentration unit 26 may have arranged in it for example silica gel orTENAX as adsorbent or absorbent. After a given concentration period,line 30 is shut and line 31, leading to the measuring chamber 22, isopened. The absorbed/adsorbed material is desorbed thermally via aheater 32 and transferred from the concentration unit 26 into themeasuring chamber 22 with the aid of the conveying pump and/or a purgegas located in cartridge 19. In certain embodiments of the invention, amoisture barrier 46, e.g. a LLDPE foil, is provided upstream of themeasurement chamber 22.

The measuring principle of a quartz microbalance, which is used inaccordance with the invention for detecting the fumigants, is known tothe skilled worker from other fields of analytics and will thereforeonly be mentioned briefly with reference to the representation of FIG.3. Each individual element of the sensor array (two of these elementsare shown in FIG. 3) exhibits a selective surface layer 31, 32, whichdiffer with regard to their sensitivity for the individual components ofthe soil air sample 33 (shown symbolically as circles 34, triangles 35or rectangles 36). For example, layer 31 is sensitive specifically forthe air components with the circles 34 as symbols, while layer 32 issensitive specifically for the air components shown as triangles 35. Achange in the mass in the individual sensor elements by the adsorptionor absorption of components of the air sample is evaluated by a changein the resonant frequency of an oscillator circuit 37, 38, which isshown schematically, via frequency counters 39, 40. However, in reality,the individual sensor elements are never one hundred percent selective.Thus, for example, when carrying out an actual measurement, sensorelement 31 will also respond to a certain extent to the air constituents35 and 36. The readings provided by the individual sensor elements 31,32 are therefore subjected to an electronic signal evaluation stage 41,which is arranged downstream and which determines the concentrations ofthe individual components in the soil-air sample 33 by means ofchemometric methods which are known per se.

Surprisingly, it has been found that supramolecular systems areparticularly suitable as selective coatings 31, 32 for detectingfumigants in air samples. In particular what are known as macrocyclesand dendrimers are attractive, owing to their monodispersivity, sincethey allow the construction of reversible, rapidly responding andregenerable gas sensors. Furthermore, owing to their cavities which canbe designed in many ways and adapted specifically to the spatialrequirements of the analyte to be detected, they allow great variety andfreedom of design. By exploiting the host-guest-interaction, to whichindividually formed hydrogen bridges also contribute, or by exploiting aspecific donor-acceptor interaction, individually tailored host systemscan be synthesized. Supramolecular host systems allow to a particularextent the ideal adaptation to the host in question since, for example,dendrimers exhibit a high tolerance for various types of functionalgroups.

FIG. 4 shows the synthetic principle of supramolecular units on thenanometer scale as an example, a guest molecule 42 acting as templatefor the circularization of two supramolecular units 43, 44 to give thehost molecule 45. The specific synthesis of attractive hosts 45 is madepossible in this manner, whose suitability as substrate-selective layersystems for the fumigants of interest in the present context can betested and optimized with the aid of the device according to theinvention.

FIG. 5 shows an example of a synthetic strategy for macrocycles, whichare also suitable as selective surface layer. The highly-flexiblesynthetic strategy for the preparation of the macrocycles has proved tobe particularly advantageous. Not only the lateral moieties A, C of whatis to become the macrocycle, but also the spacers B can be variedindependently of one another. This makes it possible to preset the typeand strength of the host-guest-interaction in a targeted fashion. Thismeans that these host molecules can be adapted without complications towhat is to become their task as sensor-active layer.

Both thermodynamic and kinetic parameters of the intercalation processesfor the adsorption and desorption of specific guest molecules can bedetermined. The information obtained can be used for the optimization orprecise differentiation between related species of guest molecules, sothat the selectivity and sensitivity of the sensor-active layers of thedevice according to the invention for the detection and thedifferentiation of related species of carbonyl compounds can be improvedsubstantially. As regards the chemistry of the macrocycles andcatenanes, reference may be made in particular to the pioneering work ofVögtle et al. Angew. Chem. 104 (1992), 1628-1631; Angew. Chem. Int. Ed.Engl. 31 (1992), 1619-1622. Synthetic strategies are also found forexample in Ottens-Hildebrandt et al. J. Chem. Soc., Chem. Commun.(1995), 777-779.

A host structure which is preorganized and capable of hydrogen bondsowing to amide groups has proved to be a particularly suitablesensor-active layer. As an example of a circular molecule meeting theserequirements, FIG. 6 shows the lactam macrocycle. The ring has fourpotential coordination centers in the form of amide, thioamide orsulfonamide groups.

Finally, preferred dendrimers for the preparation of selective surfacelayers which are suitable in particular for detecting MITC and/or1,3-dichloropropene are shown in FIGS. 7 to 12.

EXAMPLE

A device for detecting methyl isothiocyanate (MITC) in air samples wasprepared by providing six individual quartz microbalances (obtained withcommercially available electronic circuitry from HKR Sensorsysteme GmbH,Munich, Germany) with a surface layer of a lactame amide type macrocycledenoted J1. The frequency of operation or oscillation was in the rangeof around 10 MHz.

Preparation of J1

J1, i.e. a lactame amide macrocycle of the form31′-tert-butyl-5′,19′,25′,37′,40′,42′,45′,47′-octamethyl-8′,16′,28′,34′-tetraoxodispiro[cyclohexane-1,2′-[7′,17′,27′,35′,]tetraaza[10′]oxyheptacyclo[34.2.2.23′,6′.218′,21′.223′26′.111′,15′.129′,33′]-heptatetracontal[3′,5′,11′,13′,15′(41′),18′,20′,23′,25′,29′,31′,33′(46′),36′,38′,39′,42′,44′]octa-decaen-20′,1″-cyclohexane],was prepared as follows.

To a solution of 10.00 g (32.30 mmol) of1,1-bis(4′-ammo-3′,5′-dimethylphenyl)cyclohexane and 2.2 ml oftriethylamine in 50 ml of absolute dichloromethane a solution of3-phenoxyacetyldichloride in 100 ml absolute dichloromethane are wasadded dropwise at room temperature during a period of 5 hours undercontinuous stirring in a well heat-dried and argon-flushed apparatus.The resulting mixture was stirred over night, upon which the solvent wasremoved at reduced pressure using a rotary evaporator.

The residue was purified via column chromatography using silica. Uponthat, 1,4 g (37%) of a colorless solid were obtained (Mp=149-151° C.).1.20 g (1.50 mmol) thereof were dissolved together with 0.4 mltriethylamine in 250 ml of dichloromethane. Likewise, 0.39 g (1.50 mmol)of 5-tert-butyl isophthalic acid chloride were dissolved in 250 ml ofdichloromethane. Both solutions were added synchronously during a periodof 8 hours to 1 liter of the same solvent (dichloromethane) at roomtemperature. Thereafter, the reaction mixture was stirred for anadditional 2 days, upon which the solvent was removed at reducedpressure. 0.47 g (32%) of a colorless solid were obtained uponchromatographic purification on silica (Mp>260° C.). This colorlesssolid has been identified to be J1 (with a composition and structure asoutlined above) using Maldi-MS with m/z=991,2 [M⁺], 1014,2 [M⁺+Na],1030,2 [M⁺+K].

Preparation of Surface Layer

The J1 surface coatings are applied by electro-spraying a solution of J1through a capillary of an appropriately dimensioned syringe onto the topelectrode of each individual quartz microbalance(QMB), one at a time,while applying a high DC voltage of about 5 kV between the capillary andthe top electrode of the QMB. The coating process was monitored in situ.The resulting thickness of the coating was standardized by terminatingelectro-spraying when the oscillating frequency was reduced by 5 kHz.

Results

Defined concentrations of MITC were obtained by bubbling a steady anddefined stream of pure nitrogen through a melt of (MITC). Then, thestream was cooled down by means of an efficient cooling devicemaintained at a constant temperature of −12° C. Subsequently, thisdefined concentration of MITC in nitrogen was further diluted by mixingit with a defined stream of pure nitrogen to provide a stream of avariable concentration of MITC in nitrogen.

Streams having concentrations of MITC in nitrogen of 10, 25, 50, 100, or200 ppm were fed at 22 ml/min via suction through a measuring chamberhousing the QMB array. The QMBs as well as the measuring chamber weremaintained at 35° C. A reduction of oscillating frequencies of theindividual QMBs proportional to the MITC concentration was observed,namely a reduction of 2.5, 7.0, 14.0, 24.0, and 37.5 Hz, respectively.

Accordingly, MITC concentrations in the range of a few ppm can bedetected accurately and reproducibly with the device of the presentinvention.

1. A method for detecting methyl isothiocyanate in soil air samples,comprising: drawing in a soil air sample; bringing the drawn-in soil airsample into contact with at least one mass-sensitive sensor comprisingat least one quartz microbalance; detecting mass changes of the sensorin the form of electrical signals; and evaluating the electrical signalsto detect a change in mass of methyl isothiocyanate; wherein: the quartzmicro-balance is coated with a surface layer that is selective forincorporation of methyl isothiocyanate; and the quartz micro-balancecomprises macrocycles and calibrated to detect methyl isothiocyanate. 2.A method as claimed in claim 1, wherein the at least one mass-sensitivesensor comprises a plurality of sensors coated with different selectivelayers.
 3. A method as claimed in claim 1, wherein the methylisothiocyanate to be detected is concentrated before making contact withthe mass-sensitive sensor.
 4. A method as claimed in claim 1, furthercomprising determining the moisture content of the air sample.
 5. Amethod as claimed in claim 1, wherein moisture is removed from the airsample before it is brought into contact with the sensor.
 6. A method ofcontrolling phytopathogens, comprising: treating soil with an effectiveamount of a methyl thioisocyanate generating fumigant; and detectingresidual amounts of released methyl isothiocyanate by the method ofclaim 1 before proceeding with new sowings or plantings of useful plantsor crop plants in the soil.